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Minneapolis Does Storm Water Retention Right

The mile stretch of the Marquette Avenue (pictured) and Second Avenue South transit project in Minneapolis includes wider sidewalks--paved in part with permeable pavers--new transit shelters, public art and 190 new trees in iron grates.

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When it rains in Southern California the runoff, with all its contaminants and everything but the kitchen sink, ends up washing up on the beaches.

In Minneapolis all that urban clutter ends up in the Mississippi and along
its banks.

Some Southern Calif. communities are working on the storm water problem, and so is Minneapolis. Some 15,000 square feet of permeable pavers (Willow Creek Brickstone) were installed as part of the Marquette Avenue and Second Avenue South transit project in the heart of the downtown Minneapolis business district.

The storm water retention system, made by DeepRoot, comprises an underground grid of nearly 11,000 plastic-framed cells filled with a bioinfiltration soil mix. A grated cap goes over the top of the filled cells and is covered with a geotextile membrane. A layer of granite infiltration stone 2-3 inches in diameter is atop the geotextile, followed by a layer of smaller granite bedding aggregate. The permeable pavers are laid on top of the aggregate, allowing runoff to drain through the aggregate-filled voids into the soil-filled chambers below.

The interlocking concrete permeable pavers allow storm water to drain through aggregate-filled voids between the pavers into subsurface detention areas, where it's directed through a series of natural filtration processes before gradually exiting the system. The project reduces storm water runoff from nearly 5.5 acres. Marquette and Second Avenues run parallel to one another and were rebuilt from building front to building front on both sides over 12 blocks. A variety of hardwood and ornamental tress (190) were planted in a soil mix as part of the storm water drainage system. The storm water that drains through the permeable pavers and grates not only reduces runoff to the river, it also precludes the need for irrigation. Each cell can hold up to 116 cu. ft. of storm water in the uncompacted soil mix around the tree roots. Water slowly filters out from the cells through perforated pipe.

"Landscape architects (are) very interested in sustainability and best management practices," says Chris Behringer, senior urban designer with SEH in Minneapolis, a landscape architect and engineering firm. "Because storm water management is such a huge issue, permeable pavers are becoming a regular part of our process." She notes there's a "higher comfort level with pavers" than with permeable surfaces like porous concrete
or asphalt.

Through the combined use of permeable pavers and the bioinfiltration system, up to 21,600 cu. ft. of storm water from each rain event will be stored and kept from draining into the Mississippi. The system's filtration will reportedly remove over 80 percent of the phosphorus, 60 percent of total Kjeldahl nitrogen and over 90 percent of the lead, copper, zinc and iron from the storm water.

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October 13, 2019, 6:44 pm PDT

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